The present invention relates to a circulating fluidized bed reactor and method of operating the circulating fluidized bed reactor. The present invention also relates to a wall structure of a circulating fluidized bed reactor. More specifically, the invention relates to a circulating fluidized bed reactor having substantially vertical walls with cooling elements therein, the vertical walls defining the interior of the reactor chamber; means for introducing fluidization gas at the bottom of the fluidized bed reactor; means for introducing particulate material into said reactor, separator for separating particulate material from the gases, the separator being in connection with said reactor at the upper section thereof; return duct, being connected to the separator; bubbling fluidized bed adjacent to the reactor and being provided with heat exchanger means for cooling particulate material, side walls, and rear and front walls having cooling elements in fluid communication with the cooling elements of the reactor, said bubbling fluidized bed being connected with said return duct.
The present invention also relates to a method of operating a circulating fluidized bed reactor having substantially vertical walls with cooling elements therein, the vertical walls defining the interior of the reactor chamber; means for introducing fluidization gas at the bottom of the fluidized bed reactor; means for introducing particulate material into said reactor; separator for separating particulate material from the gases, the separator being in connection with said reactor at the upper section thereof; bubbling fluidized bed adjacent to the reactor and being provided with heat exchanger means for cooling particulate material, side walls, and rear and front walls having cooling elements in fluid communication with the cooling elements of the reactor, a discharge channel between said heat exchanger and rear wall; the method comprising the steps of maintaining a circulating fluidized bed in the reactor by providing entrainment of a substantial amount of particulate material from the reactor to the separator; separating particulate material from the gas in the separator and returning separated material back to the reactor; introducing particulate material into the bubbling fluidized bed above the upper surface of bubbling fluidized bed; fluidizing the particulate material in the bubbling fluidized bed and recovering heat from the fluidized particulate material by said heat exchanger; discharging the cooled particulate material from said bubbling fluidized bed from the lower section thereof into the lower section of a discharge channel; fluidizing said discharged particulate material in said discharge channel and introducing the particulate material from the upper section of said discharge channel into the reactor.
U.S. Pat. No. 5,060,599 shows a circulating fluidized bed reactor having pockets formed in the side wall thereof to receive material flowing downwardly along the wall. The pocket is provided with an upward opening at a location where the density of the fluidized bed is considerably lower than that adjacent to the reactor bottom. This document shows how to control the material flow by allowing the material to outflow over the edge of the pocket or by discharging material via a duct or opening in the bottom of the pocket. The pocket is formed inside the reactor by providing a partition wall in the reaction chamber. To have a sufficient volume for the pocket and heat transfers therein the partition wall must be considerably high. A heavy wall structure of this kind is very difficult as it causes stresses to other structures at its joining points and also undesirable vibration of structures. If the height of the partition wall is increased, the operation of such a pocket will be restricted to merely high load operations. At low loads, insufficient amounts of solid material will be falling into the pocket. Also, since the pocket may be emptied directly via the opening at its bottom, there must be some additional means for controlling the discharge of the material and for preventing any accidental discharge thereof.
U.S. Pat. No. 4,716,856 shows an integral fluidized bed heat exchanger in an energy producing plant. There is shown an integral fluidized bed heat exchanger and fluidized bed reactor having a common wall between them. The common wall is provided with openings for allowing the material from the fluidized bed heat exchanger to overflow into the reactor. As disclosed, there must be separate controlling facilities and a recycle leg for directing the surplus material separated from the gases directly back to the reactor. This arrangement has only one level from which the material overflows to the reactor. The gases and particles flow through the same opening.
In U.S. Pat. No. 4,896,717 there is shown a fluidized bed reactor in which a recycle heat exchanger is located adjacent to the furnace of the reactor with each enclosing a fluidized bed and sharing a common wall which includes a plurality of water tubes. In this document, the solids are also suggested to overflow back to the reactor. However, this document suggests to direct all separated material via the recycle heat exchanger back to the reactor. This results in that the capacity of the recycle heat exchanger must such as to allow the material to flow even at a maximum load, which easily leads to an unnecessarily large and over-dimensioned construction with regard to the performance of the heat exchanger. Also, the fluidization gas of the recycle heat exchanger must be conveyed via the overflow opening and further downwardly in the passage to the reactor.
U.S. Pat. Nos. 5,069,170 and 5,069,171 show also integral recycle heat exchangers in connection with a circulating fluidized bed reactor. Those, however, apply several compartments in the external heat exchanger chamber to manipulate the solids flow. The initial principle of introducing solid material from the bed to the reactor is also an overflow of material. These solutions are somewhat complicated.
In EP publication 0 550 932 there is shown a system for cooling hot particulate material from a fluidized bed reactor having three distinct fluidized beds in an external, separate fluidized bed cooler. The material entrained with the gases is separated from the exhaust gases and is directed to a first fluidized bed from which the material is facultatively directed either to a second fluidized bed or a discharge duct. The second and a third fluidized bed cooler are located adjacently, below the first fluidized bed being divided by a common wall and communicating with their lower and upper sections. There is a gas space above the second and the third fluidized bed coolers and below the first fluidized bed to collect and pass the gas and solids to the common discharge duct connecting the fluidized bed cooler with the reactor. In this arrangement, it is difficult to efficiently control the flow of solids due to the general layout. It is also highly potential that a short circuit of hot solids is formed, i.e., solids flow easily uncooled from the first fluidized bed directly to the discharge duct.
U.S. Pat. No. 4,363,292 discloses an arrangement for providing heat transfer sections on the bottom grid of a fluidized bed reactor. In this system, there are also partition walls above the grid which divide the bottom section of the reactor into several sections. This arrangement has also a limited capability to provide sufficiently of heat transfer surface in the heat transfer section, particularly for low load conditions. This and other known methods of operating a fluidized bed reactor still have shortcomings which the present invention aims to abolish.
It is an object of the present invention to provide a circulating fluidized bed with an integrated compact heat exchanger, which solves the problems of the prior art.
It is a further object of the present invention to provide a circulating fluidized bed with an integrated compact heat exchanger, which efficiently complies with the demands on the heat exchange rate.
It is still a further object of the present invention to provide a wall structure partitioning the integrated compact heat exchanger and the circulating fluidized bed reactor.
It is still a further object of the present invention to provide a wall structure partitioning the integrated compact heat exchanger and the circulating fluidized bed reactor, which may be utilized as a part of a particulate material discharge channel.
It is still a further object of the present invention to provide a compact fluidized bed heat exchanger, which has a high mixing rate of particulate material and a reliable material circulation/return system.
It is still a further object of the present invention to provide a compact fluidized bed heat exchanger, which has a self-adjusting bed level control.
It is still a further object of the present invention to provide a compact fluidized bed heat exchanger which has a compact and efficiently supported partition wall with a main reactor.
For meeting these and other objects of the invention, the circulating fluidized bed reactor of the present invention according to its first aspect includes substantially vertical walls with cooling elements, the walls defining the interior of the reactor chamber; means for introducing fluidization gas at the bottom of the fluidized bed reactor; means for introducing particulate material into said reactor; separator for separating particulate material from the gases, said separator being in connection with said reactor at the upper section thereof; return duct connected to the separator; a bubbling fluidized bed adjacent to the reactor and provided with heat exchanger means for cooling particulate material, side walls, and rear and front walls having cooling elements in fluid communication with the cooling elements of the reactor, said bubbling fluidized bed being connected to said return duct, and the circulating fluidized bed comprising a solid tight discharge channel, i.e. a channel disabling movement of particulate material through its walls, between said heat exchanger means and the rear wall, for discharging material from the bubbling fluidized bed to the reactor, and a lower opening section in said discharge channel for allowing particulate material to come from the bottom section of the bubbling fluidized bed and enter the lower section of the discharge channel, the upper opening section in said discharge channel allowing particulate material to be discharged from the upper section of the discharge channel into the reactor.
Preferably the particulate material in said discharge channel is maintained at a fluidized state so that it is in a flowable form and readily controllable. There may be independently controllable fluidization gas introduction means for both the discharge channel and the bubbling fluidized bed. According to the invention, the particulate material is directed from above the bubbling fluidized bed to its reactor side half. The introduced particulate material may be hot solids directly from the circulating fluidized bed or from the separator which separates solids from the reactor exhaust gases.
According to a preferred embodiment of the present invention, the lower opening of the discharge channel is located vertically below the upper portion of the heat exchanger and the upper opening of the discharge channel is above the lower portion of the heat exchanger, so that at least a portion of the heat exchanger is immersed in the bubbling fluidized bed. According to the invention, the discharge channel consists of several distinct, individual small channels for creating the required cross-sectional area on the first hand, and a robust, cooled structure on the other. The cross section of the individual channel is preferably rectangular, but naturally this may be arranged also in a different manner, still gaining at least some of the advantages of the present invention. The discharge channel or several channels are preferably so dimensioned as to have an areal cross section &lt;30%, preferably &lt;20% of the cross section of the bubbling fluidized bed.
According to another aspect of the present invention, the circulating fluidized bed reactor with substantially vertical walls with cooling elements therein, the vertical walls defining the interior of the reactor chamber, includes means for introducing fluidization gas at the bottom of the fluidized bed reactor; means for introducing particulate material including fuel into said reactor; separator for separating particulate material from the gases, said separator being in connection with said reactor at the upper section thereof; bubbling fluidized bed provided with a heat exchanger for cooling particulate material, said bubbling fluidized bed having side walls and a rear wall having cooling elements in fluid communication with the cooling elements of the reactor, a front wall structure partitioning the bubbling fluidized bed and the circulating fluidized bed from each other, the front wall consisting essentially of substantially vertical tubes being formed in a manner to provide at least one discharge channel within said wall structure including at least one substantially vertical solid tight portion, i.e, a portion substantially disabling penetration of particulate material through it, for transferring particulate material, said discharge channel being capable of discharging solids from the lower section of said bubbling fluidized bed and introducing the same into the circulating fluidized bed. Advantageously the discharge channel comprises an opening from the lower section of the discharge channel to the lower section of said bubbling fluidized bed, i.e, a lower opening, and an opening from the upper section of the discharge channel to the reactor, i.e, an upper opening. Also it is preferred to arrange the lower opening below the upper portion of the heat exchanger, and the upper opening is above the lower portion of the heat exchanger to ensure that at least a portion of the heat exchanger is immersed in the bubbling bed. The discharge channel is preferably formed in the wall by bending the tubes away from the discharge channel area and turning them behind the tube adjacent to or outside said area.
A method of operating a circulating fluidized bed reactor is provided according to the present invention, in connection with a circulating fluidized bed reactor having substantially vertical walls with cooling elements therein, said vertical walls defining the interior of the reactor chamber; means for introducing fluidization gas at the bottom of the fluidized bed reactor; means for introducing particulate material into said reactor; separator for separating particulate material from gases, said separator being in connection with said reactor at the upper section thereof; bubbling fluidized bed adjacent to the reactor and being provided with heat exchanger means for cooling particulate material, side walls, and rear and front walls having cooling elements in fluid communication with the cooling elements of the reactor, a discharge channel between said heat exchanger and rear wall; the method comprising the steps of maintaining a circulating fluidized bed in the reactor by providing entrainment of substantial amount of particulate material from the reactor to the separator, separating particulate material from the gas in the separator and returning the separated material back to the reactor; introducing particulate material into the bubbling fluidized bed above the upper surface of the fluidized bed therein; fluidizing the particulate material in the bubbling fluidized bed and recovering heat from the fluidized particulate material by said heat exchanger; discharging cooled particulate material from said bubbling fluidized bed at the lower section thereof into the lower section of the discharge channel; fluidizing said discharged particulate material in said discharge channel and introducing particulate material from the upper section of said discharge channel into the reactor. Advantageously the upper surface of the bubbling fluidized bed is maintained at least on the same vertical level as the particulate material is introduced from the upper section of said discharge channel into the reactor.